The present invention relates to lighting units that display images by adjusting the amount of light to be transmitted, and to display equipment that uses these lighting units.
The display equipment generally used can be broadly divided into a light-emitting type, which is represented by a cathode-ray tube (CRT) display or a plasma display panel (PDP) display, and a non-light-emitting type, which is represented by a liquid-crystal display (LCD), an electrochromic display (ECD), or an electrophotoretic image display (EPID).
The non-light-emitting type mentioned above has a display panel (such as an LCD panel) that adjusts the transmittance of light, and a lighting unit provided at the rear of the display panel in order to radiate light thereto. That is to say, the display panel displays images by adjusting the amount of transmission of the light radiated from the lighting unit. Hence, the lighting unit has a very important meaning in image display.
The lighting unit in the non-light-emitting type of display equipment employs either the edge-light scheme (light-guiding plate scheme), the directly downward lighting scheme (reflection plate scheme), the plane-like light source scheme, or the like. (Bibliography: “Liquid-Crystal Display Technology,” pp. 252-256, published on Nov. 8, 1996 by Sangyo Tosho K.K., and “Full-Color Liquid-Crystal Display Technology,” pp. 201-202, published on Feb. 26, 1990 by K.K. Toriqueps). The edge-light scheme or the directly downward lighting scheme is mainly used for medium-size and larger types of LCD equipment.
Also, a hybrid scheme, a combination of the above-mentioned edge-light scheme and directly downward lighting scheme, has been proposed in recent years.
Examples of existing lighting units employing the hybrid scheme are shown in FIGS. 28 and 29. The general configuration of hybrid-scheme lighting units is explained using the example of FIG. 28. A light unit of the hybrid scheme is composed principally of reflection plate 450, a plurality of light sources 110a to 110c arranged in parallel on the reflection plate, a plurality of light-guiding plates 290a to 290d arranged along the longitudinal direction of the light sources, a plurality of semi-transmitting reflection means 605a to 605c arranged directly above the light sources, and light diffusion means 740 provided on the entire plane of the light radiating side. This configuration makes it possible under the hybrid scheme, as with the directly downward lighting scheme, to achieve a larger screen and higher luminance by increasing the number of light sources, and at the same time, to maintain the uniformity in the in-plane distribution of luminance by providing light-guiding plates. It is therefore possible under the hybrid scheme to implement easily a lighting unit more uniform in the luminance of light than under the directly downward lighting scheme, and higher in luminance than under the edge-light scheme. Technology relating to lighting units of the hybrid scheme is set forth in, for example, Japanese Application Patent Laid-Open Publication No. Hei-208631 (1990), Hei-214191 (1991), Hei-338723 (1992), Hei-282921 (1997), and Hei-149073 (1999).
By the way, liquid-crystal display (LCD) equipment consisting of such a lighting unit and an LCD panel poses the problem that resolution decreases during full-motion image display mode. According to “Technical Report on Signal Engineering”, EID96-4, pp.19-26, 1996, released by Ishiguro et al., the problem mentioned above is ascribed to the fact that the image display speed of the LCD panel, or the response speed of the liquid crystals, is low (several tens of milliseconds), in other words, that a display scheme, called the hold type, is employed. In this Report, one frame means one cycle of time in video signals. How and why image quality deteriorates during full-motion image display of the hold type is described as follows:
In normal real-image display mode, a moving object, for example, constantly moves and does not stop in the same position. In hold-type display mode, however, since even a moving object continues to be displayed at the same position during one entire frame, although the image at the proper position is displayed during a moment of one frame, an image different from the real image continues to be displayed during another moment. The human eye perceives these images by averaging them, and thus resolution decreases. Technology for solving this problem is reported in IDRC'97, pp. 203-206, 1998, issued by K. Sueoka et al. This technology makes it possible for image quality deterioration due to such averaging to be minimized for improved full-motion image quality by blinking the lighting unit and displaying an image only at a specific moment.
In general, a maximum luminance of at least 400 cd/m2 is required for television-use display equipment to achieve dynamic image quality. Therefore, to attain such a luminance level using a non-light-emitting type of display equipment such as LCD equipment, its lighting unit needs to have a high luminance falling within the range from at least 5,000 to 8,000 cd/m2, since the fact that, although this depends on resolution and/or display mode, the light transmittance of the LCD panel usually ranges from about 5 to 8 percent, must be considered. And to achieve a luminance of at least 5,000 cd/m2 using a hybrid-scheme lighting unit, there is a need to use a cold cathode-ray tube (cold cathode fluorescent lamp), which is the mainstream in light sources, to use a tube current from 5 to 6 mA in order to allow for longer light source life, and to arrange a plurality of such light sources at intervals of about several tens of millimeters (or several centimeters) in parallel. This means a further increase in the number of light sources required, and accordingly, leaves pending or poses the problem that cannot be solved by achieving the uniformity of light with the conventional light-guiding plate. This problem is described below.
Conventional lighting units of the hybrid scheme can be divided into two major types: a type with flat light-guiding plates, and a type that uses plate-shaped members of the wedge type in its cross sectional structure.
FIGS. 28 and 29 are schematic block diagrams (partly cross-sectional diagrams) of lighting units of the hybrid scheme; one lighting unit using flat light-guiding plates, and one lighting unit using wedge-type light-guiding plates.
FIG. 28 shows a lighting unit that uses flat light-guiding plates 290a to 290d, and FIG. 29 shows a lighting unit that uses wedge-type light-guiding plates 290a to 290d. Wedge-type light-guiding plates 290b and 290c in FIG. 29 are butted to the respective thin edges.
First, problems associated with the structure of a lighting unit having flat light-guiding plates are described below.
As exemplified in FIG. 28, since the light-guiding plates have a flat shape, part of the light (for example, beam 1010 in the figure) that has been radiated from the light source (here, attention is paid to light source 110a in the figure) and has entered the corresponding light-guiding plate leaks to the light source side located next (here, light source 110b in the figure). Part of the light that has leaked is absorbed or dissipated by the phosphor of the light source located next, and as a result, the light is lost and cannot be used for lighting. For a flat light-guiding plate, therefore, the problem remains unsolved that since the rate of radiated light to the light which can be used for lighting is small (namely, the utilization efficiency of the source light is low), high luminance cannot be easily obtained, and this problem also is more significant for light sources arranged at shorter intervals. Therefore, this problem cannot be ignored since it is particularly significant in the lighting unit of TV-use LCD equipment that requires an light source arrangement interval of about several tens of millimeters.
Next, problems associated with the structure of a lighting unit having wedge-type light-guiding plates are described below.
Compared with the lighting unit of FIG. 28, the lighting unit in FIG. 29 that includes light-guiding plates having a wedge-type butted plate-shaped members in their cross-sectional structure decreases in the amount of light lost by the light source located next. For this reason, the lighting unit shown in FIG. 29 offers higher luminance than that achievable using the lighting unit whose light-guiding plates are flat in shape. However, at portion 10002 (the thinnest portion) of each light-guiding plate, since its cross section has a discontinuous portion, the luminescent lines corresponding to this discontinuous portion occur and reduces the uniformity in the in-plane distribution of luminance. In addition, the distribution of light in its traveling direction, after being radiated along the light-guiding plate, changes at the thinnest portion 10002, and along with this, the distribution of light in its traveling direction, after being radiated from the lighting unit, also changes. That is to say, there occurs the problem that the repetition of changes in contrast at the boundary between the thinnest portion 10002 of the light-guiding plate and the portion directly above the light source, namely, the nonuniformity of luminance, is made perceivable to the persons 5000a and 5000b who observe from diagonal directions.
This means that for the conventional lighting units of the hybrid scheme, design considerations relating to the simultaneous achievement of the high luminance and in-plane uniformity of luminance required for the implementation of LCD equipment suitable for TV use are not contained, these requirements are difficult to satisfy at the same time.